Nonaqueous liquid electrolyte-type electrical double-layer capacitors can be charged and discharged at a high current, and thus hold considerable promise as energy storage devices for such applications as electric cars and auxiliary power supplies.
Prior-art nonaqueous liquid electrolyte-type electrical double-layer capacitors are constructed of a nonaqueous liquid electrolyte and positive and negative polarizable electrodes composed largely of a carbonaceous material such as activated carbon. The composition of the nonaqueous liquid electrolyte is known to have a large influence on the withstand voltage and electrostatic capacitance of the capacitor.
The nonaqueous liquid electrolyte is composed of an electrolyte salt and a nonaqueous organic solvent. Studies have hitherto been conducted on various combinations of such electrolyte salts and nonaqueous organic solvents.
For example, quaternary ammonium salts (e.g., JP-A 61-32509, JP-A 63-173312, JP-A 10-55717) and quaternary phosphonium salts (e.g., JP-A 62-252927) are commonly used as the electrolyte salt because of their solubility and degree of dissociation in organic solvents, as well as their broad electrochemical stability range.
Examples have also been reported in which dialkylimidazolium salts, which are ionic liquids, are used as the electrolyte salt (JP-A 6-61095, JP-A 2002-110472).
However, in electrical double-layer capacitors where solid quaternary salts are used as the electrolyte salt, the quaternary salt readily deposits out of solution at low temperatures, and particularly at very low temperatures of −20° C. or less. Even in the absence of such deposition, the electrical conductivity falls off dramatically at low temperatures.
When dialkylimidazolium salts, as ionic liquids, are used to resolve this problem, mixed systems of these salts with inorganic salts are very sensitive to such factors as humidity in the air, and are thus difficult to handle. Moreover, imidazolium salts themselves have drawbacks; namely they have melting points which are not as low as might be desired, and they also have a relatively narrow potential window.
The polarizable electrodes are generally composed of activated carbon. This activated carbon is made by carbonizing any of various suitable starting materials, examples of which include natural substances such as coconut shells and sawdust, synthetic resins such as phenolic resins and polyimide resins, and also coal- and petroleum-based pitch, mesophase carbon, carbon fibers and discarded tires. The carbonized material is then activated, such as by gas activation with steam or carbon dioxide, or by chemical activation using zinc chloride, potassium hydroxide or phosphoric acid. The larger the specific surface area of the activated carbon, the greater the electrostatic capacitance tends to be, although this relationship has yet to be fully investigated.
It is therefore one object of the invention to provide electrical double-layer capacitors endowed with excellent low-temperature characteristics and a high electrostatic capacitance.